Bent glass sheets are used extensively for vehicle windshields, side windows, and rear windows. To improve the mechanical strength of the glass, the bent glass sheets are also frequently tempered which additionally causes the glass to break into relatively small, dull pieces rather than into relatively long, sharp pieces. In the United States, tempered bent glass sheets are used on vehicle side and rear windows while annealed bent glass sheets laminated to each other by polyvinyl butyral are used for vehicle windshields. In other countries, tempered bent glass sheets are used for vehicle windshields as well as side and rear windows.
Glass sheet press bending is performed by pressing a heated glass sheet between complementary curved molds so that the heated glass sheet is bent to conform to the curved shape of the molds. One type of press bending system includes a horizontal conveyor on which glass sheets are conveyed in a generally horizontally extending orientation for the heating and also includes an upper mold that is located above the conveyor at a bending station. A lower mold is moved upwardly from below the conveyor to lift each heated glass sheet upwardly toward the upper mold for the press bending operation whereupon a vacuum is drawn at the upper mold to secure the glass sheet as the lower mold is moved downwardly. Thereafter, a transfer mold is moved horizontally under the upper mold and receives the press bent glass sheet for subsequent transfer therefrom. Normally, the transfer mold is formed as an open center ring and transfers the press bent glass sheet to a quench station where tempering is performed. This type of press bending system can be utilized with either a gas hearth or roller type conveyor. However, with a gas hearth conveyor where the glass sheets are conveyed on a thin film of pressurized gas, a groove has to be provided in the hearth to permit the lower mold to move downwardly below the conveyor so that the heated glass sheet can be conveyed over the lower mold in preparation for the upward movement of the lower mold for the press bending operation. Likewise, a roller conveyor utilizing this type of press bending system requires that the lower mold be of the segmented type so as to be movable upwardly between the spacings between the rolls and full engagement with the periphery of the glass sheet being pressed is thus not possible.
Prior art references disclosing the type of press bending system described above include U.S. Pat. Nos. 3,607,187; 3,607,200; 4,092,141; 4,260,408; 4,260,409; 4,265,650; 4,272,275; 4,290,786; and 4,430,110.